Purpose: Spatially-selective arterial spin labeling perfusion MRI is sensitive to arterial transit times (ATT) that can result in inaccurate perfusion quantification when ATTs are long. Velocity-selective ASL is robust to this effect because blood is labeled within the imaging region, allowing immediate label delivery. However, velocity-selective ASL cannot characterize ATTs, which can provide important clinical information. Here, we introduce a novel pulse sequence, called VESPA ASL, that combines velocity-selective and pseudo-continuous ASL to simultaneously label different pools of arterial blood for robust cerebral blood flow (CBF) and ATT measurement.
Methods: The VESPA ASL sequence is similar to velocity-selective ASL, but the velocity-selective labeling is made spatially-selective and pseudo-continuous ASL is added to fill the inflow time. The choice of inflow time and other sequence settings were explored. VESPA ASL was compared to multi-delay pseudo-continuous ASL and velocity-selective ASL through simulations and test-retest experiments in healthy volunteers.
Results: VESPA ASL is shown to accurately measure CBF in the presence of long ATTs, while ATTs < TI can also be measured. Measurements were similar to established ASL techniques when ATT was short. When ATT was long, VESPA ASL measured CBF more accurately than multi-delay pseudo-continuous ASL, which tended to underestimate CBF.
Conclusion: VESPA ASL is a novel and robust approach to simultaneously measure CBF and ATT and offers important advantages over existing methods. It fills an important clinical need for non-invasive perfusion and transit time imaging in vascular diseases with delayed arterial transit.
